This invention relates to an improved method and apparatus for making a stamping master to facilitate video disk replication.
The prior art methods currently in use generally rely on a photoresist technique. In this type of process, a photoresist coating sensitive to, for instance, laser or ultraviolet light is applied to a substrate. Selected areas of the coating, which correspond to the information to be encoded on the disk, are then subjected to the laser beam. The laser beam alters the areas of the coating upon which it impinges making those areas more soluble to developing solvents. Those areas of the photoresist coating which were subjected to the laser light are removed while the areas of the photoresist coating not impinged upon by the light remain unaffected. A thin conductive layer is then deposited over the developed photoresist and subjected to an electroplating bath. After plating to an appropriate thickness, the plated material and the conductive layer are separated from the substrate and photoresist and used as a stamping master.
The master is used as a die to form pits in a transparent plastic base material. Next a thin reflective layer of aluminum is deposited over the plastic base material and a protective coating is placed over the aluminum layer. Finally, two such assemblies are sandwiched together to form a double sided video disk.
In information retrieval, a light beam penetrates the transparent plastic base and is focused on the surface of the aluminum reflective layer. Light reflected from inside a pit will destructively interfere with light reflected from the spaces between the pits. Thus, intensity modulation of the reflected light beam is achieved as the video disk rotates and this intensity modulation is translated back into the stored information.
The prior art methods for the manufacture of a video disk are involved and costly processes. The primary disadvantage of these processes is that they provide no means by which the accuracy of the information etched onto the master may be verified until the conductive layer is formed on the partially completed master. Only then can the accuracy be verified. Obviously, any mistakes beyond a certain limit render the master worthless. Errors cannot be corrected. Moreover, a constant, costly supply of photoresist material and processing chemicals and facilities are required.
The method of the present invention on the other hand allows the coded information to be read and therefore verified for accuracy, and even corrected or erased, before the formation of the stamping master is even begun. This is achieved by using a material which is reversibly convertible between two stable states to initially receive the coded information. This information can be read for accuracy before forming the actual master.
One group of materials which exhibits variations in structure is amorphous semiconductors. Such materials are described and illustrated, for example, in U.S. Pat. No. 3,530,441 by Stanford R. Ovshinsky issued Sept. 22, 1970 and U.S. Pat. No. 3,678,852 by Julius Feinleib, et al., issued July 25, 1972. These amorphous semiconductor materials can be switched between two stable states in response to the application of light, electromagnetic energy or heat. In one state the material is substantially disordered, generally amorphous and has a high resistance or insulating condition. In the other state the atomic structure of the material is changed to a different local order as, for example toward a more ordered crystalline condition having a comparatively low resistance or conducting characteristic. For the purposes of this invention one of these states will be called hereinafter the crystalline or high conductivity state and the other will be called the amorphous or low conductivity state.
Energy is selectively applied to discrete portions of the semiconductor material causing an alteration in those discrete portions from the crystalline to the amorphous state or vice versa. The condition of those discrete portions can then be detected with respect to the surrounding areas to allow retrieval of the stored information in a non-destructive manner.
Furthermore, the method of the present invention obviates the need for the aforementioned conductive layer which is used in the prior art. This reduction in steps contributes a further savings of both time and expense as compared to the prior art.